The proposed experiments will investigate the relationship between intrauterine positioning relative to female or other male fetuses during prenatal life and adult reproductive performance in male mice. Individual variation in the reproductive performance of adult female mice has previously been found to relate to their former intrauterine proximity to male fetuses during prenatal life. The results of two recently completed studies provide evidence that (1) as a normal consequence of multiple uterine residence, male mouse fetuses that develop in utero between female fetuses (2F males) are exposed to significantly higher levels of estradiol than are males that develop in utero between other male fetuses (0F males); and (2) in adulthood, 2F males are significantly more aggressive than (and dominant over) 0F males when placed in direct competition. I propose to utilize 0F and 2F males to examine the effects of intrauterine positioning on adult behaviors that could influence a male mouse's reproductive success in the wild. Specifically, aggressiveness of adult 0F and 2F males will be compared when they are (1) placed in direct competition, (2) paired with a standard olfactory bulbectomized male opponent, and (3) castrated at birth, administered testosterone in adulthood, and paired with a standard, bulbectomized male opponent. Copulatory behavior (both hetero and homotypic) will be compared in adult 0F and 2F males when they are (1) paired with sexually receptive females and (2) castrated at birth, administered estradiol alone or estradiol plus progesterone in adulthood and paired with a sexually experienced standard male. This last experiment will thus provide a measure of the sensitivity of 0F and 2F males to both estrogen and progesterone when adult. Finally, the tendency to exhibit pup killing (infanticide) toward unfamiliar newborn pups and the rate of urine marking in a novel environment will be compared in adult 0F and 2F males.